1. Field of the Invention
This invention relates to apparatus for overcoming the problem known as "tingle voltage", in which differential potentials occurring due to ground current flows in the vicinity of a structure having an electrical supply system result in low level electric shocks to persons or livestock exposed to those differential potentials.
2. Review of the Art
The problems associated with tingle voltage phenomena are discussed in U.S. Pat. No. 4,573,098 (Williston). This patent proposes a solution to those problems, which involves the insertion of a device known as a tingle voltage filter, consisting primarily of a saturating inductor, normally having high alternating current impedance but having a low impedance under fault conditions, into the neutral to ground connection of an installation to be protected. The purpose of the filter is to ensure that under normal conditions, its impedance will be high compared with that of the ground current flow paths across which tingle voltage may be developed, so that potential differences are developed mainly across the filter rather than across the ground flow paths. Under fault conditions in which a large potential is applied to the filter, the inductor will saturate, reducing its impedance so that a large current flows and normal circuit protection devices can operate. This filter has proved successful in practice in overcoming tingle voltage problems. There are however certain fault conditions which could theoretically result in substantial and sustained tingle voltages occurring in the presence of the tingle voltage filter. These conditions are in general associated with wiring errors which should not but nevertheless could occur. There is also a slight possibility of certain low level i.e. high impedance, faults occurring which might also tend to result in higher tingle voltages with a tingle voltage filter present than would occur otherwise.
In all of these fault conditions, the problem derives from a sustained current which flows into the bonding ground of the protected structure. The current is limited to a low enough value that the normal circuit.. protective devices do not operate, usually by the impedance of an electrical appliance. In an installation not containing a tingle voltage filter, this current would be able to return to the power system neutral directly through the neutral to ground connection in the distribution panel. With a filter however, the impedance of the filter causes a bonding ground potential rise and tingle voltages, greater than would otherwise occur. The inductor in the filter can be constructed such that when it is saturated, the impedance of the filter is low enough that the tingle voltage increase is acceptably small. However, at the times during which this current is changing directions, the inductor becomes unsaturated for a brief interval, which is small in comparison to the period of the power supply. During this interval, the inductor's impedance is high, and so most of the power supply voltage at that time will appear across the filter. Where this current is highly reactive or highly capacitive, the power supply voltage will be near its peak of 170 volts at these times and therefore the tingle voltages may contain peaks approaching this value. In spite of this high peak magnitude, due to the short duration of the peaks, a root-mean-square voltage measurement of the tingle voltage under these conditions would yield an acceptably small value. Although the root-mean-square measurement technique is commonly used for measurement of non-sinusoidal waveforms, there exists no experimental evidence to justify the assumption that humans or livestock are responsive to root-mean-square voltage, and the significance of peak voltage is undetermined.
A further problem which has hampered acceptance of the tingle voltage filter of the Williston patent is its very unobtrusiveness during normal operation, since it has no means of providing assurance to a user that it is indeed operative.